1. Field of the Invention
The present invention relates to a standard and non-standard video signal identifying apparatus for determining whether a composite video signal input to a television receiver or the like is a standard signal or a non-standard signal.
The above-mentioned standard signal refers to a television signal in which the frequency of the color subcarrier signal is strictly managed so as to be exactly an integral multiple of the frequency of the vertical synchronizing signal like a broadcast wave (composite video signal) from a broadcasting station. The non-standard signal refers to a television signal in which the frequency of the color subcarrier signal is a non-integral multiple of the frequencies of the vertical synchronizing signal and the horizontal synchronizing signal like a playback video signal (composite video signal) output from a digital video tape recorder, a digital video camera, a digital still camera or other video apparatuses.
2. Description of the Prior Art
In television broadcast signals, the frequency of the color subcarrier signal is strictly managed so as to be exactly an integral multiple of the frequency of the vertical synchronizing signal. Therefore, television receivers are configured as described below in order to effectively use the relationship between the color subcarrier frequency and the vertical frequency and that the vertical synchronizing signal can be precisely played back even when the electric field is weak or when a disturbance occurs due to ghosts. That is, one burst signal is extracted from an input video signal, a clock of a frequency of axc3x97fSC (a is an integer not less than 1, and fSC is the color subcarrier frequency) is generated based on a color burst signal, and the clock is divided, thereby playing back the vertical synchronizing signal.
In the above-mentioned dividing operation, when a standard signal such as a broadcast wave is handled, vertical playback can be normally performed because standard signals satisfy.
Vcount=(axc3x97fSC/fH)xc3x97(N/2).
Here, Vcount is the number of clocks of one field section, fH is the horizontal frequency, and N is the number of scanning lines of one frame.
However, when a non-standard signal like a signal obtained in special playback of a VTR (video tape recorder) is handled, the dividing operation cannot be normally performed because the following relationship holds for non-standard signals:
xe2x80x83Vcountxe2x89xa0(axc3x97fSC/fH)xc3x97(N/2).
Therefore, in the case of a non-standard signal, vertical playback is performed by separating the vertical synchronizing signal from the video signal by a synchronizing separator circuit.
Moreover, in television receivers, a processing such as Y/C separation using a frame comb filter or inter-field interpolation is used to enhance the image quality. When the Y/C separation is used, in the case of standard signals in which the color subcarrier frequency is exactly an integral multiple of the vertical frequency, the luminance signal and the color signal can be precisely separated by the frame comb filter, so that the effect of enhancing the image quality is expected.
However, in the case of non-standard signals in which the color subcarrier frequency is a non-integral multiple of the vertical frequency, the luminance signal and the color signal cannot be separated precisely, so that the above-mentioned image quality enhancing processing rather degrades the image quality. Therefore, it is desirable that the image quality enhancing processing be performed only in the case of standard signals and be not performed in the case of non-standard signals.
In order that vertical synchronization playback is not performed by the synchronizing separator circuit or the image quality enhancing processing is not performed when the input video signal is a non-standard signal, it is necessary to detect that the video signal is a non-standard signal.
Now, the configuration and operation of a conventional apparatus for identifying standard and non-standard video signals will be described.
FIG. 7 is a block diagram showing a concrete example of the conventional standard and non-standard video signal identifying apparatus in the case of generation of the vertical synchronizing signal of an NTSC television receiver.
A reset circuit comprising an AND gate 401 outputs a reset signal when an external reset signal supplied from a microcomputer comes in or when an external vertical synchronizing signal separated by a synchronizing separator circuit (not shown) comes in. The reset signal is input to a reset terminal R of a vertical counter 402. Moreover, to the vertical counter 402, a clock of a frequency of axc3x97fSC generated by a clock generating circuit 403 is input to a clock terminal CLK. The clock generating circuit 403 generates the clock of a frequency of axc3x97fSC based on a color burst signal extracted from the video signal.
The vertical counter 402 counts the number of clocks of one field section after the reset signal is input. A load hold flip-flop 404, which receives the output of the vertical counter 402 as the input signal and uses the external vertical synchronizing signal as the load hold pulse, latches the value that the vertical counter 402 indicates when the external vertical synchronizing signal comes in, and outputs the latched value. That is, the output of the load hold flip-flop 404 represents the clock count of one field section defined by the external vertical synchronizing signal.
In a field clock count setting register 405, a value corresponding to the clock count of one field section of a standard signal is set. A subtracter 406 subtracts the value set in the field clock count setting register 405 from the output of the load hold flip-flop 404. The output value of the subtracter 406 corresponds to the shift of the clock count value of one field section defined by the external vertical synchronizing signal from the clock count value corresponding to the standard signal. Then, the output of the subtracter 406 is converted into an absolute value by an absolute value circuit 407 and the absolute value of the shift is output. In a threshold value setting register 408, a threshold value for identifying standard and non-standard signals is set. A comparator 409 compares the output of the absolute value circuit 407 with the threshold value set in the threshold value setting register 408. When the output of the absolute value circuit 407 is not more than the threshold value, the input video signal is determined to be a standard signal, and when the output exceeds the threshold value, the input video signal is determined to be a non-standard signal.
The basic idea of the above-described conventional apparatus for identifying standard and non-standard video signals is as follows: In the case of the NTSC system, the input video signal is determined to be a standard signal when the clock count Vcount of one field section satisfies the following relationship:
xe2x88x92Mxe2x89xa6{Vcountxe2x88x92(axc3x97fSC/fH)xc3x97(N/2)}xe2x89xa6M(M greater than 0)
where M is the threshold value for identifying standard and non-standard signals. In other cases, the input video signal is determined to be a non-standard signal.
However, in the conventional apparatus for identifying standard and non-standard video signals, the identification of standard and non-standard signals is performed based on the clock count of only one field section. Therefore, a non-standard signal such as the output of a digital still camera in which the clock count is shifted by one clock (clock frequency axc3x97fSC) in n field sections although the shift of the clock count of one field section is smaller than one clock and Vcount satisfies the above-mentioned relationship in the unit of one field is erroneously determined to be a standard signal.
For this reason, although the output of a digital still camera is a non-standard signal, a playback vertical synchronizing signal obtained by dividing the clock of a frequency of axc3x97fSC is used for playing back the image. Consequently, vertical synchronization is shifted every n fields because the output is actually a signal in which the clock count is shifted by one clock in n field sections as mentioned above.
Accordingly, an object of the present invention is to provide an apparatus for identifying standard and non-standard video signals with which even a non-standard signal having only a slight shift and being conventionally erroneously determined to be a standard signal in the unit of one field can be determined to be a non-standard signal.
An apparatus for identifying standard and non-standard video signals according to a first aspect of the invention comprises clock generating means, clock counting means, resetting means, count value holding means, first determining means, subtracting means, selecting means, and second determining means.
The clock generating means has the function of generating a clock of a frequency of axc3x97fSC (a is an integer not less than 1, and fSC is a color carrier frequency) based on a color burst signal extracted from a video signal.
The clock counting means whose maximum output value is a value corresponding to the number of clocks generated by the clock generating means in one field section of a standard signal has the function of counting the number of clocks generated by the clock generating means and repetitively changing an output value thereof from zero to the maximum output value.
The resetting means has the function of resetting a count value of the clock counting means to zero in response to input of an external reset signal and canceling the reset of the count value of the clock counting means in response to an external vertical synchronizing signal separated from the video signal and input after the input of the external reset signal.
The count value holding means has the function of holding the count value of the clock counting means in response to the input of the external vertical synchronizing signal.
The first determining means has the function of comparing an output value of the count value holding means with a first threshold value to thereby determine whether the output value of the count value holding means is zero or a value in the vicinity thereof, or the maximum output value of the clock counting means or a value in the vicinity thereof.
The subtracting means has the function of obtaining a difference between the output value of the count value holding means and the maximum output value of the clock counting means.
The selecting means has the function of, based on a result of the determination by the first determining means, selecting the output value of the count value holding means when the output value of the count value holding means is zero or a value in the vicinity thereof, and selecting the output value of the subtracting means when the output value of the count value holding means is the maximum output value of the clock counting means or a value in the vicinity thereof.
The second determining means has the function of determining that the video signal is a non-standard signal when an absolute value of an output value of the selecting means is higher than a second threshold value.
According to this configuration, in a system using a signal being a constant multiple a of the color carrier frequency fSC as the system clock, the clock count of the clock counting means returns to zero every time its output reaches the maximum output value, that is, every time the number of clocks is counted to the value corresponding to the clock count of one field section of the standard signal, and the clock counting means is not dependent on the incoming external vertical synchronizing signal. Consequently, the shift of the clock count of one field section is accumulated over a multiplicity of fields, so that even a signal in which the shift is not less than one clock in the unit of n fields although the shift is smaller than one clock and the relationship as the standard signal is satisfied in the unit of one field like the output of a digital still camera can be determined to be a non-standard signal.
In the above-described configuration, the clock counting means comprises, for example, a counter for counting the number of clocks generated by the clock generating means, a field clock count setting register for setting a value corresponding to the number of clocks output by the clock generating means in one field section of the standard signal, and a decode circuit for outputting a coincidence signal when a count value of the counter coincides with the set value of the field clock count setting register, and supplying the coincidence signal to a reset terminal of the counter.
By thus configuring the clock counting means, in a system using a signal being a constant multiple a of the color carrier frequency fSC as the system clock, only when the output of the vertical counter becomes the value corresponding to the clock count of the one field section of the standard signal which is the value set by the field clock count setting register, the vertical counter is reset so that its count value returns to zero, and is not dependent on the incoming external vertical synchronizing signal. Consequently, the shift of the clock count of one field section is accumulated over a multiplicity of fields, so that even a signal in which the shift is not less than one clock in the unit of n fields although the shift is smaller than one clock and the relationship as the standard signal is satisfied in the unit of one field like the output of a digital still camera can be determined to be a non-standard signal.
Moreover, in the above-described configuration, while it is desirable that the first threshold value be one-half the maximum output value of the clock counting means, the threshold value is not limited thereto. It may be any value that is higher than the maximum value of values in the vicinity of zero estimated to be the output value of the count value holding means and is lower than the minimum value of values in the vicinity of the maximum output value in the case of a non-standard signal.
The count value holding means comprises, for example, a load hold flip-flop receiving the external vertical synchronizing signal as a load hold input and receiving the clock generated by the clock generating circuit as a clock input. The resetting means comprises a set-reset flip-flop receiving the external reset signal as a set input, and receiving the external vertical synchronizing signal as a reset input.
An apparatus for identifying standard and non-standard video signals according to a second aspect of the invention comprises clock generating means, pulse thinning out means, clock counting means, count value holding means, n field clock count setting means, subtracting means, and determining means.
The clock generating means has the function of generating a clock of a frequency of axc3x97fSC (a is an integer not less than 1, and fSC is a color carrier frequency) based on a color burst signal extracted from a video signal.
The pulse thinning out means has the function of thinning out an external vertical synchronizing signal separated from the video signal, and outputting the thinned out signal once every n fields (n is a plural number).
The clock counting means has the function of counting the number of clocks generated by the clock generating means, and its count value is reset to 0 in response to an output pulse of the pulse thinning out means.
The count value holding means has the function of holding an output value of the clock counting means in response to the output pulse of the pulse thinning out means.
The n field clock count setting means has the function of setting a value corresponding to the number of clocks generated by the clock generating means in n field sections of a standard signal.
The subtracting means has the function of obtaining a difference between an output value of the count value holding means and the set value of the n field clock count setting means.
The determining means has the function of determining that the video signal is a non-standard signal when an absolute value of an output value of the subtracting means is higher than a predetermined threshold value.
According to this configuration, in a system using a signal being a constant multiple a of the color carrier frequency fSC as the system clock, the clock counting means counts the number of clocks of n field sections, the output value of the clock counting means produced immediately before the clock counting means is reset is held by the count value holding means, and the output value of the count value holding means is compared with the value corresponding to the clock count of n field sections of the standard signal set by the n field clock count setting means. Consequently, even a signal in which the shift is not less than one clock in the unit of n fields although the shift is smaller than one clock and the relationship as the standard signal is satisfied in the unit of one field can be determined to be a non-standard signal.
An apparatus for identifying standard and non-standard video signals according to a third aspect of the invention comprises clock generating means, burst extraction pulse generating means, burst extracting means, a band-pass filter, a latch pulse generating means, a holding means group, subtracting means, and determining means.
The clock generating means has the function of generating a clock of a frequency of bxc3x97fH (b is a predetermined integer, and fH is a horizontal frequency) based on a horizontal synchronizing signal separated from a video signal. It is desirable that the integer b be not less than 500.
Now, why it is desirable that the integer b be not less than 500 will be explained. In television broadcasting, the horizontal frequency fH is 15.734264 kHz, and the burst frequency fSC is 3.579545 MHz. In the relationship between these two frequencies, the color burst signal is latched (data-held) at a predetermined timing, defined by a clock, within the period of the color burst portion represented by the burst extraction pulse. That is, one clock section of a sine wave of 3.579545 MHz is latched. Therefore, it is necessary that the frequency of the clock be not less than 3.579545 MHz. When b=500, bxc3x97fH=7.875 MHz, and the clock frequency is approximately twice the burst frequency. It is considered that the clock frequency is a value significant for comparing the held data after n fields.
The burst extraction pulse generating means has the function of generating a burst extraction pulse corresponding to a period of a color burst portion of the video signal once every field based on the external horizontal synchronizing signal and an external vertical synchronizing signal separated from the video signal, and the clock.
The burst extracting means has the function of extracting from the video signal a signal portion corresponding to an output period of the burst extraction pulse.
The band-pass filter whose center frequency is a burst frequency has the function of passing therethrough a burst frequency component of an output signal of the burst extracting means.
The latch pulse generating means has the function of generating, based on the burst extraction pulse and the clock, a latch pulse at a predetermined timing, defined by the clock, within the period of the color burst portion represented by the burst extraction pulse.
The holding means group comprises a number, (n+1), (n is a plural number) of holding means connected in series. The holding means hold an output of the band-pass filter in response to the latch pulse and transmit the output to succeeding holding means.
The subtracting means has the function of obtaining a difference between an output signal of the first holding means of the holding means group and an output signal of the (n+1) th holding means.
The determining means has the function of determining that the video signal is a non-standard signal when an absolute value of an output value of the subtracting means is higher than a predetermined threshold value.
According to this configuration, in a system using a signal being a constant multiple b of the horizontal frequency fH as the system clock, even a signal in which the shift is one clock in the unit of n fields although the relationship as the standard signal (Vcount≈bxc3x97N/2; N is the number of scanning lines of one frame, and N/2 is the number of scanning lines of one field) is satisfied in the unit of one field can be determined to be a non-standard signal.
That is, the level of the color burst signal at a timing, defined by the clock, within the period of the burst extraction pulse is detected in two fields being n fields away from each other, and the difference between the levels in the two fields is compared with the threshold value. Therefore, there is hardly any shift of the phase of the color burst signal between two consecutive fields, so that there is hardly any difference in the level of the color burst signal. Consequently, even in a case where a non-standard signal is erroneously determined to be a standard signal, since the phase shift of the color burst signal is accumulated and the difference in the level of the color burst signal is consequently accumulated, the signal can be determined to be a non-standard signal.
While the latch pulse generating means generates a latch pulse, for example, at the center of the period of the color burst portion represented by the burst extraction pulse, the latch pulse may be generated at any timing other than the center that is defined by the clock of a frequency being a constant multiple b of the horizontal frequency fH. The holding means comprises a load hold flip-flop receiving the latch pulse as a load hold input and receiving the clock as a clock input.